Open system crystal fractionation and incompatible element variation in basalts

O'HARA1 has recently proposed an open-system model for the fractional crystallisation of magmas involving continual replenishment of the magma chamber and only partial extraction of the fractionated liquid at each stage. This could produce a series of chemically uniform lavas (such as basalts) with very little resemblance to the primary mantle melt. It is claimed that the model can also explain large variations in the concentrations of incompatible trace elements and their inter-element ratios which would otherwise be ascribed to variable degrees of partial melting or inhomogeneity in the mantle source region. The purpose of this note is to explore the potential and limitations of O'Hara's model in this respect and to test it against the observed variability of Icelandic basalts.